Method for the physico-chemical treatment of effluents, in particular of surface water for consumption

ABSTRACT

A process is disclosed for the physico-chemical treatment of effluent, especially surface water intended for consumption, the process includes the successive steps of coagulation, flocculation and settling, wherein, during the flocculation step, a ballast is introduced into the sludge, the function of which ballast is to make the sludge heavier. The contacting mass used during this flocculation step consists of part of the densified sludge resulting from the settling step and continuously recycled in the flocculation step, without any treatment.

FIELD OF THE INVENTION

The present invention relates to a process for the physico-chemicaltreatment of effluent, especially of surface water intended forconsumption.

BACKGROUND OF THE INVENTION

It is known that the use of physico-chemical processes is common to mosttreatments applied to various types of water and that these treatmentsessentially consist of:

clarification of surface water for consumption or for industry;

clarification of municipal sewage, storm water or industrial wastewater;

decarbonization;

removal of phosphates;

etc.

These types of physico-chemical treatments always comprise the followingsuccessive steps:

coagulation: a step of neutralization of the colloids using a metalsalt, generally a trivalent iron or aluminium compound, in order to forma microfloc.

This coagulation step may be carried out in one or more steps;

flocculation: a step of agglomeration and growth of the microfloc. Thisagglomeration step takes place by virtue of the addition of apolyelectrolyte (or polymer) downstream of the coagulation step;

settling: a step of separation of the floc from the interstitial water,causing the formation of sludge on the one hand, and of clarified wateron the other hand.

Over the last thirty years or so, the state of the art relating to sucha physico-chemical treatment has evolved considerably as a result of theappearance of two technologies:

flocculation with a contacting mass, which has allowed the quality ofthe flocs to be improved, the volume of the reactors to be reduced andthe clarification to be improved. This is because the microflocs of thecoagulation have a greater chance of agglomerating and of growing as thereaction medium contains a high density of particles: the rate of flocformation is proportional to the number of free particles in thesuspension;

lamellar settling, carried out by introducing inclined plates or tubesin the settling tanks. This technology has made it possible to reducethe size of the settling tanks by from 50 to 70%

The current technological trend is towards improving the flocculationconditions, which are key in determining the quality of the treatedwater and in obtaining high settling velocities.

At the present time, modern settling tanks use two types of contactingmasses in the flocculation reactor:

1. recirculated presettled sludge: an example of this technique isdescribed in FR-A-2,553,082;

2. fine ballasts, such as microsand: an example of the use of thistechnique is described in FR-P-1,411,792 and in FR-A-2,627,704.

The advantages and disadvantages of the two known techniques indicatedabove, of flocculation with a contacting mass, will now be explained.

1. Flocculation Using Sludge as the Contacting Mass

FIG. 1 of the appended drawing shows diagrammatically a physico-chemicaltreatment plant employing this technique. This figure showsdiagrammatically, at A, the coagulation reactor, at B, the flocculatorand, at C, the settling tank. These are plants well known to thoseskilled in the art and, under these conditions, they will not bedescribed.

Thus, as may be seen in this FIG. 1, the contacting mass in theflocculation reactor B consists of the recirculation of part of thesludge which has settled in C. The recirculated sludge volume representsbetween 0.5 and 4% of the treated volume. The excess, concentratedsludge is extracted and removed. Their volume represents between 0.1 and1% of the treated volume, depending on the treatments.

The advantages of this flocculation technique using recirculatedpresettled sludge as the contacting mass are the following:

the contacting mass is generated by the process, and is thereforeavailable without any quantity limitation, depending on the requirementsof the process;

the contacting mass presents a very high specific surface area orspatial occupation because of its expanded structure and its lowrelative density; by way of example, 1 gram of flocculated sludge in oneliter (average concentration in the reactor) occupies, after settlingfor approximately 5 minutes, a volume equal to 100 ml.

This very high specific surface area or spatial occupation considerablyincreases the probability of contact between the flocs and the very fineparticles, coagulated colloids and micro-organisms, and therefore of“trapping” this suspended matter very efficiently.

The drawbacks and limitations of this technique involve the settlingspeeds obtained with densified sludges which are between 30% and 80% ofthe velocities obtained with ballast.

2. Flocculation Using a Ballast as the Contacting Mass

According to this technique, the contacting mass is obtained by adding,upstream of a flocculator, a fresh or recycled ballast after cleaning.The means making it possible to separate and regenerate the ballastwhich is to be recycled in the flocculator are means well known to thoseskilled in the art and, under these conditions, they will not bedescribed.

When implementing this technique, the ballast generally consists of sandand the continuously extracted materials amount to approximately 5% ofthe volume of water treated by the settling tank; these extractedmaterials, laden with sludge coating the microsand, must be treated soas to regenerate the sand; the cleaned sand is subsequently reinjectedupstream of the flocculator, at the front of the plant. The residuegenerated by this sand-ballast cleaning operation represents the excesssludge.

It will be noted that the existing ballast-type apparatus described inthe literature, and especially in FR-P-1,411,792 and in FR-A-2,627,704,include a ballast-recycling step for obvious running-cost reasons.Moreover, in all the documents describing this technology it isspecified that the ballast is always “cleaned”, i.e. regenerated. Thisis because, the ballast, “coated” with the polymer, must have themaximum area of adhesion for the precipitation flocs produced chemicallyduring coagulation. An effective physical cleaning is thereforeindispensable for maximizing the binding area available.

The ballast is often sand, generally having a diameter of between 50 μmand 150 μm, usually called microsand.

The publication Journal Water SRT-AQUA, Vol. 41, No. 1, pp. 18-27, 1992describes a curve relating the turbidity of the water produced to thediameter of the ballast particles, which demonstrates that this processbecomes effective when the sand particles do not exceed 150 μm, theresults being even better with values of the order of 50 to 100 μm.

It should be pointed out that the advantage of this technique offlocculation using a contacting mass consisting of a fine ballastessentially resides in the settling velocity, which may be from 20% to200% greater than the velocities obtained by the flocculation processesusing a contacting mass consisting of recirculated presettled sludge.Thus, when clarifying river water, the indicated velocities through thelamellar modules are between 25 and 50 m³/m².h, while the equipmentimplementing the flocculation process using sludge as the contactingmass is limited to velocities of between approximately 15 and 30m^(3/)m².h.

The essential drawbacks of this technique mainly stem from the fact thatthe ballast must provide two different functions:

accelerated flocculation, by virtue of the use of a contacting masshaving a high specific surface area (or spatial occupation);

increase in the settling velocities, resulting from the addition ofballast to the floc.

These limitations or drawbacks can be imputed to the followingcharacteristics:

for equivalent contacting mass (by weight), the ballast offers acontacting surface area or percentage of spatial occupation which ismuch less than the sludge. By way of example:

in the case of “flocculation with sludge”, the concentration in thereactor is approximately 1 g/l and the volume occupied by the sludgeafter five minutes of settling is approximately 10% of the initialvolume;

in the case of “flocculation with ballast (for example sand)”, theballast concentration in the reactor should reach at least 5 g/l, whilethe volume occupied by sludge after five minutes of settling is onlyapproximately 1% of the initial volume;

increasing the amount of ballast, desirable for obtaining a highcontacting mass (and not for obtaining a high settling velocity), leadsto an increase in the volume of sludge recirculated to theextracted-sludge treatment system, which treatment consists inseparating the sludge from the sand so as to regenerate the latter. Thisoperation is generally carried out by hydrocyclones supplied at highpressures, which operation therefore becomes very expensive from anenergy consumption standpoint. In fact, and so as to limit the runningcosts, the volume of recirculated sludge is intentionally limited tobetween 5 and 10% of the volume treated and the ballast concentration inthe reactor does not exceed 5 to 10 g/l: quite obviously this choice isincompatible with the possibility of optimizing the flocculation.

various techniques are aimed at compensating for the deficit in thecontacting mass resulting from the operating conditions described above,such as:

the use of additional flocculation energy (figures ranging up to 100times the conventional flocculation energy may be mentioned) or

the use of even finer ballast particles, increasing the specific surfacearea (for example, particles having a diameter of between 10 and 50 μm),is not conceivable, on the one hand, for energy cost reasons and, on theother hand, for reasons of difficulty in settling and in sand-flocseparation.

In summary, the performance characteristics of flocculation with ballastare limited by three factors:

the system is sensitive to sudden pollution caused by lack ofavailability of binding sites on the ballast (the contacting mass islimited to a maximum of 5-10 g/l);

the system has a lower performance with regard to so-called “sensitive”pollutants (helminth eggs, microorganisms, microparticles, traces ofcomplex organic compounds, pesticides etc.);

the low concentration of extracted sludge, resulting from the need toclean the ballast as fully as possible—this concentration is at least 10times lower than that measured on apparatus using a sludge contactingmass—and frequently involving the installation of a complementary unit,downstream of the settling tank, for thickening the extracted sludge.

BRIEF DESCRIPTION OF THE INVENTION

In view of the drawbacks and limitations of the conventional processesmentioned above, the aim of the invention is to provide a novel processmaking it possible to combine the advantages of the efficiency offlocculation employing a contacting mass consisting of densifiedrecirculated sludge with the high settling rates of a flocculationprocess using ballast.

Consequently, this invention relates to a process for thephysico-chemical treatment of effluent, especially of surface waterintended for consumption, comprising the successive steps ofcoagulation, flocculation and settling, characterized in that, duringthe flocculation step, a ballast is introduced into the sludge, thefunction of which ballast is to make the sludge heavier, and in that thecontacting mass used during this flocculation step consists of part ofthe densified sludge resulting from the settling step and continuouslyrecycled in the flocculation step, without a washing operation.

Thus, the process according to the invention employs a ballast, but itis used differently from that in the so-called “flocculation withballast” process described above. According to the invention, theballast has only one role, that of ladening, and the flocculationfunction of the contacting mass is performed just by the recirculatedsludge. The ballast no longer constitutes a surface for adhesion,offered to the particles, but simply constitutes a ladening mass whichbecomes incorporated into the sludge recirculated to the flocculationreactor, this recirculated sludge constituting the contacting mass.

According to one characteristic of the present invention, the ballastconsists of a material having a particle size of between 50 and 500 μm,preferably between 100 and 300 μm.

According to a preferred embodiment of the invention, this ballast is adense inorganic material (actual particle density between 2 and 8 g/ml),especially sand, garnet or magnetite.

According to the present invention, the excess, densified sludge, whichis not recycled, may be discharged without treatment or else may betreated so as to recover the ballast, this treatment not including athorough cleaning of the ballast separated from the sludge. In the caseof discharge without treatment, the densified sludge exhibits superiorsettling capability.

According to the invention, the ballast is preferably recovered bygravity sedimentation, either inside or outside the settling tank, therecovered ballast then being recycled in the flocculation step.

Other features and advantages of the present invention will emerge fromthe description given below with reference to the appended drawing whichillustrates one example of implementation, this being devoid of anylimiting character.

In the drawings:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram illustrating the principle relating to the knownphysico-chemical treatment process described above, in which thecontacting mass consists of recirculated presettled sludge;

FIG. 2 is a diagram, similar to FIG. 1, illustrating the principle ofthe process which forms the subject of the present invention.

DETAILED DESCRIPTION

As may be seen in this FIG. 2, the contacting mass in the flocculator Bconsists of the continuous recirculation of part P1 of the densifiedsludge, after settling and thickening in the settling tank C, butwithout any separation or regeneration operation. According to apreferred method of implementation of this process, the recirculatedvolume represents between 0.5% and 4% of the treated volume.

A fraction P2 of the sludge, densified in excess, must obviously beremoved, for example with a purge volume of between 0.1% (clarification)and 1% (waste water with a very high content of suspended matter) of thetreated volume of water. At this stage, two comments should be made:

depending on the level of purge and the cost of the ballast, the excesssludge may be either purely and simply discharged, or treated in orderto recover the ballast.

sludge treatment, for recovering the sand, is different from that of theballast-induced flocculation (FR-P-1,411,792 and FR-A-2,627,704). Thisis because, according to the invention, the sand is recovered withoutbeing cleaned, i.e. without regeneration, and the treatment is performedon concentrated sludge since it is not necessary to clean the sandthoroughly in the separation phase.

Shown diagrammatically at E in FIG. 2 are the means provided accordingto the invention for recovering the ballast. It will be noted that thelow volume of sludge to be extracted, 0.1% to 1% of the water volumetreated (i.e. from 5 to 50 times less than in the case of flocculationwith ballast), makes it possible, optionally, to use separationtechniques which are more sophisticated and have a higher performance,among which may be mentioned, in particular:

separation by hydrocyclone,

separation by blowing air,

separation by elutriation,

separation by centrifuging,

separation by ultrasound,

or to reduce the energy cost of this separation station.

Given that the ballast merely has a simple ladening role and does nothave the function of a contacting mass, the size of the particles ofthis ladening ballast, con- trary to the case of ballast-inducedflocculation, may advantageously be chosen towards larger diameters.Thus, it is possible to use weighting particles having a diameter ofbetween 50 and 500 μm and preferably between 150 and 300 μm, while inthe case of flocculation using a contacting mass consisting of aballast, the particle size of the latter must be less than 150 μm andpreferably between 50 and 100 μm.

This ballast diameter, in the process which forms the subject of theinvention, constitutes a fundamental characteristic in order to:

increase the settling velocities (if d=100 μm, the ballast settlingvelocity=30 m/h, while if d=250 μm, the ballast settling velocity=115m/h);

recover the ballast from the extracted sludge.

It will be noted that, in some cases, if the ballast diameter is largeenough, simple gravity separation, inside or outside the settling tank,may be envisaged in order to be able to recover and recycle the ballast.

The process according to the invention, with flocculation using acontacting mass consisting of recirculated densified sludge, has inparticular the following advantages:

1. It provides a very high percentage of spatial occupation of thebinding material, hence the treatment is highly effective:

stability of the purification performance characteristics, even whensignificantly increasing the charge of raw water;

capability of thorough removal of so-called “sensitive” pollutants(microparticles, micro-organisms, traces of complex organic compounds,pesticides, helminth eggs, etc.).

2. The ballast only has a ladening function. Its average particle sizemay therefore be greater than that required for ballast-inducedflocculation (e.g. 250 μm as opposed to 100 μm). This possibility hastwo advantages:

the settling velocities are considerably increased, this being the moreso as the diameter of the ladening material is greater;

recovery of the ballast is all the more easy as its diameter is greater.

In some cases, if its diameter is sufficiently large, it is possible toenvisage a simple gravity separation inside or outside the settlingtank.

3. The recovered ballast does not need to be cleaned since it is notdesired to regenerate “clean” sites for the coagulation/flocculation.Consequently, it is possible, and advantageous, to recover the ballastfrom highly concentrated sludge, which possibility has the followingadvantages:

since the extracted sludge is roughly 10 times more concentrated, thevolume of the thickener/storage unit installed downstream of thesettling tank may be proportionately reduced;

the sand-recovery system operates on smaller volumes in the sameproportions (e.g. 10 times smaller) and, here too, the size of theequipment and the energy consumption may be reduced.

4. Separating the flocculation sludge-mass/ballast functions makes itpossible to envisage operating with a low throughput (between less than20% and 80% of Qmax, depending on the case) without adding orrecirculating the ballast, thereby making it possible to reduce therunning costs even further.

The table below summarizes the comparative performances of the processforming the subject of the present invention and of the processes of theprior act mentioned above. This table refers to these processes beingcarried out on river water, the treatment volumes being identical foreach implementation.

TABLE Type of Flocculation Flocculation Flocculation with apparatus withsludge with ballast densified sludge Velocity in laminar 25 40 60settling modules (m/h) Coagulation and 10 10 10 flocculation time(minutes) Ballast: No Yes Yes - diameter 50-100 μm 200 μm -concentration 5 g/l 3 g/l Amount of recirculation  2  7  2 (%) InfluentSM 5 to 70 5 to 70  5 to 70 (in mg/l) Treated water: - turbidity (NTU)0.5 to 1   1 to 5  1 to 2 - SM (mg/l)   <2 2 to 10 2 to 5 Sludgeconcentration _30 g/l ≦3 g/l _30 g/l (g/l)

Of course, it remains the case that the present invention is not limitedto the embodiments described and illustrated, rather it encompasses allalternative forms thereof which fall within the scope of the appendedclaims.

What is claimed is:
 1. A physico-chemical process for treating effluentcomprising the steps: subjecting the effluent to coagulation for formingmicroflocs; subjecting the coagulated effluent to flocculation foragglomerating the microflocs into flocs; subjecting the resultingflocculated effluent to settling for separating flocs and interstitialwater components thereby forming sludge and clarified water; recycling aportion of the sludge to the flocculated effluent during the step offlocculation, the recycled sludge serving as a contact mass for thefloc; and adding a ballast to the coagulated effluent during the step offlocculation thereby densifying the sludge, wherein a portion of thedensified sludge is recycled, after settling, to the flocculatedeffluent during the flocculation step without cleaning the ballast,wherein said ballast consists of a material having a particle size of atleast about 200 μm, and wherein the volume of densified sludgeconstituting the contacting mass, which is recirculated, after settling,to said flocculation step, is between 0.5 and 4% of the effluent volumetreated.
 2. Process according to claim 1, wherein said ballast consistsof a material having a particle size of between 200 and 500 μm. 3.Process according to claim 2, wherein said ballast is a dense inorganicmaterial, having an actual particle density between 2 and 8 g/ml. 4.Process according to claim 1, wherein part of the densified sludge isnot recycled in the flocculation step and is extracted in the settlingstep, and represents a purge volume of about 0.1 to 1% of the volume ofwater treated.
 5. Process according to claim 4, wherein the excess,densified sludge, which is not recycled, is discharged withouttreatment.
 6. Process according to claim 4, wherein the excess,densified sludge, which is not recycled, is treated so as to recover theballast inside or outside a settling tank, said treatment not includinga removal of the ballast from the sludge.
 7. Process according to claim6, wherein the ballast is recovered by gravity sedimentation, inside thesettling tank, the recovered ballast then being recycled in saidflocculation step.
 8. Process according to claim 6, wherein the ballastis recovered by gravity sedimentation, outside the settling tank, therecovered ballast then being recycled in said flocculation step.